Glass for thermal shock-resistant beverage containers

ABSTRACT

The invention relates to a glass having the composition (in % by weight, based on oxide) SiO 2  about 78.5-about 79.5, B 2 O 3  about 13.0-about 14.0, Al 2 O 3  about 2.0 -about 3.0, Na 2 O about 4.5-about 5.5, K 2 O 0-about 0.6, which may be used in the production of thermal shock-resistant beverage containers, particularly teapots, coffee machine jugs and baby-milk bottles.

FIELD OF THE INVENTION

[0001] The invention relates to glass, and more particularly, to a glassthat may be used in the production of thermal shock-resistant beveragecontainers.

BACKGROUND OF THE INVENTION

[0002] Glass containers intended for the preparation or storage of hotbeverages, such as, for example, coffee machine jugs, teapots andbaby-milk bottles, should be made of glasses having high thermal shockresistance, which arises from a low coefficient of thermal expansion anda low modulus of elasticity, and good chemical resistance. Such vesselsare therefore made of borosilicate glasses, which may be used forlaboratory equipment.

[0003] The group of borosilicate glasses has been known for some time.For example, German patent specifications DE 588 643 and DE 679 155disclose heat-resistant glasses made from SiO₂, Al₂O₃, B₂O₃ and R₂O, inparticular from (% by weight) ≧80 SiO₂,13 B₂O₃, 2 Al₂O₃ and 4 Na₂O,having a coefficient of expansion α_(20/300) of ≦3.4 ·10⁻⁶/K.Borosilicate glasses for laboratory applications must meet strictrequirements and satisfy the DIN ISO 3585 standard on “Borosilicateglass 3.3”, i.e., must have, inter alia, a coefficient of linear thermalexpansion α_(20/300) of between 3.2 and 3.4·10⁻⁶/K.

[0004] Owing to their composition, the known glasses which comply withthe above standard have very high melting points. In addition, they canonly be produced with comparatively low melting capacities. Whileconventional container glasses based on soda-lime glass are produced inequipment having melting capacities of up to 450 tons of glass per daywith maximum temperatures below 1450 ° C., melting capacities of lessthan 60 tons of glass per day are usual for borosilicate glasses 3.3 andmelting points of at least 1650° C. are necessary. One reason for thelow melting capacities is glass melting furnaces for larger throughputscannot be built since no materials are available for constructing, forexample, large domes for the high temperatures. Another reason is thatrelatively large electric glass melting furnaces cannot guaranteeuniform heating. Owing to the smaller equipment and higher meltingpoints, the production of these borosilicate glasses requiressignificantly more energy than does the production of soda-lime glasses.This, together with the more expensive raw materials for borosilicateglasses, results in higher glass prices for borosilicate glasses 3.3.

[0005] Against the background of increasing pressure on industry to saveenergy and to reduce production costs overall, the use oflow-melting-capacity energy-intensive borosilicate glass 3.3 can nolonger be justified for products which do not have to satisfy the verystrict requirements of laboratory equipment. At the same time, however,the energy saving and productivity increase achieved must not be negatedby plant down times during the glass change of production of analternative glass in the same melting equipment.

SUMMARY OF THE INVENTION

[0006] One feature of the invention is, therefore, to find a glass whichrequires less melting energy, i.e., a glass having low melting andworking points, had adequate thermal shock resistance for the productionof heat-resistant beverage containers, and has high chemical resistancesimilar to that of borosilicate glasses 3.3.

[0007] This feature can be achieved by using a glass as describedherein.

[0008] A glass from the narrow composition range (in % by weight, basedon oxide) of SiO₂ about 78.5-about 79.5 B₂O₃ about 13.0-about 14.0 Al₂O₃about 2.0-about 3.0 Na₂O about 4.5-about 5.5 K₂O 0-about 0.6

[0009] Owing to the balanced ratio of the components present, combinesproperties which were hitherto regarded as uncombinable with oneanother.

[0010] The relatively high SiO₂ facilitates the low thermal expansion;at even higher contents, the improved melting properties, expressed bythe reduced melting point, would not be achieved.

[0011] Al₂O₃ in the stated amounts counters phase separation of theglass, which would result in a reduction in the chemical resistance andin haze. At least about 2.0% by weight are desirable for this purpose.Desirably, higher contents than about 3.0% by weight should not becombined with the other requirements of a glass because the meltingpoint may rise to an impermissible extent.

[0012] The relatively high content of Na₂O can cause the reduction inthe melting point. This action can be reinforced further by a K₂Ocontent of up to about 0.6% by weight.

[0013] The narrow range mentioned for the B₂O₃ content, together withthe alkali metal oxide(s), can produce the low melting point. HigherB₂O₃ contents may result in a significant increase in the raw materialscosts, which can negate the savings achieved by the lower melting energyrequirement. Lower contents are likewise not desirable because this canresult in a rise in the melting point. In principle, a lowering of themelting point could be achieved by a further increase in the alkalimetal content, but, desirably, the stated upper limits for Na₂O and K₂Oare not exceeded in order to satisfy the high demands on chemicalresistance. With a lower alkali metal content than the stated lowerlimit, the lower melting point may not be achieved owing to therestriction in the B₂O₃ content.

[0014] In order to improve the glass quality, the glass can also containconventional fining agents, such as As₂O₃, Sb₂O₃ or chlorides (NaCl,KCl) in conventional amounts, such as from about 0.1 to about 2 weightpercent. It is furthermore possible for the glass to contain up to atotal of about 0.5% by weight of further oxides, such as, for example,MgO, or CaO oxides which may be introduced into the glass compositionvia impurities and which have no interfering effect, i.e., do notadversely influence the suitability for the stated use. It is alsopossible for decolorants, such as, for example, Er₂O₃ or CoO, to beincluded, which counteract or hide the coloring effect of iron which isusually present in the raw materials.

[0015] The glass used in accordance with the invention has a workingpoint V_(A), i.e., the temperature at a viscosity of about 10⁴ dPas, of≦ about 1220° C., and preferably, the working point is within about+/−10° C. of about 1210° C. This temperature is below that of thecommercially available borosilicate glass 3.3 having the composition (in% by weight) 80.l SiO₂, 13.0 B₂O₃, 2.5 Al₂O₃, 3.5 Na₂O, 0.6 K₂O, 0.3NaCl (See Comparitive Example V described hereinafter) with a workingpoint V_(A) of 1250° C. The improvement is even clearer on comparison ofthe temperatures at a viscosity of 10³ dPas (T3), which is of greaterrelevance for melting of the glass. For the glass according to theinvention, this temperature is at most about 1460° C., while it is 1530°C. for Comparative Example V.

[0016] The figures document the ease of melting of the glass. It enablesthe maximum melting point to be lowered by about 30° C. in industrialmelting units with a simultaneous increase in the production capacity byabout 10%, in each case compared with V of Example 1.

[0017] It is known that the chemical resistance, in particular thehydrolytic and acid resistance, is impaired for a glass whosecomposition is varied by reducing the SiO₂ content and increasing thealkali metal content so that the glass becomes “softer”, i.e., itsmelting point is reduced.

[0018] Surprisingly, this was not the case in the present invention.Instead, the chemical resistance of the glass is very high. The glasshas both a hydrolytic resistance H in accordance with DIN ISO 719 inhydrolytic class 1 and an acid resistance S in accordance with DIN 12116 in acid class 1. Its caustic lye resistance L in accordance with DINISO 659, in lye class 2, is just as good as for borosilicate glass 3.3.This is particularly surprising inasmuch as the glass, compared with theglass V of Example 1, contains more Na2O, which is known for itsdisadvantageous effect on the chemical resistance, and no additionalcomponents, such as, for example, CaO, for improving the hydrolytic andacid resistance.

[0019] The glass has a coefficient of linear thermal expansionα_(20/300) of between about 3.5 and about 3.7·10⁻⁶/K and a modulus ofelasticity E of ≦ about 65 GPa. Preferably, the modulus of elasticity isas low as possible, such as below about 65 GPa. With these properties,the glass has a low specific thermal stress φ, which is given byφ=(E·α)/ (1−μ), where μ is the Poisson number, which hardly changes atall with the glass composition and can be assumed to be a constant valueof about 0.2. Thus, the glass according to Working Example A (asdescribed below) has a specific thermal stress φ− about 0.3 MPa/K, whileφ for conventional soda-lime container glass (α=9.0·10⁻⁶/K, E=70 GPa) is0.78 MPa/K.

[0020] The specific thermal stress is a measure of the thermal shockresistance. With this low specific thermal stress, the glass has asufficiently high thermal shock resistance for it to be eminentlysuitable for many purposes, including beverage container glass,particularly baby-milk bottles, coffee machine jugs and teapots, withthe thermal shocks that occur in these applications.

[0021] Without further elaboration, it is believed that one skilled inthe art can, using the preceding description, utilize the presentinvention to its fullest extent. The following preferred specificembodiments are, therefore, to be construed as merely illustrative, andnot limitative of the remainder of the disclosure in any way whatsoever.

[0022] In the foregoing and in the following examples, all temperaturesare set forth uncorrected in degrees Celsius; and, unless otherwiseindicated, all parts and percentages are by weight.

[0023] The entire disclosure of all applications, patent andpublications, cited above and below, and of corresponding GermanApplication No. 199 13 227.5-27, filed Mar. 23, 1999, is herebyincorporated by reference.

E X A M P L E S Example 1

[0024] The Table depicts a glass from the composition range according tothe invention (Working Example A) and a Comparative Example V, with therespective compositions (% by weight) and properties.

[0025] After the raw materials had been weighed out and mixedthoroughly, the glasses were melted in an electrically heated meltingunit, which may be a conventional melter, at temperatures of up to 1620°C. (A) or 1650° C. (V). TABLE Composition (in % by weight) andproperties of a working example (A) and a comparative example (V): A VSiO₂ 79.0 80.1 B₂O₃ 13.45 13.0 Al₂O₃ 2.4 2.5 Na₂O 4.85 3.5 K₂O — 0.6NaCl 0.3 0.3 α_(20/300) [10⁻⁶/K.] 3.6 3.3 Glass transition temperature530 520 T_(g) [° C.] V_(A) [° C.] 1205 1250 T3 [° C.] 1440 1530 E [GPa]64 63 H [class] 1 1 S [class] 1 1 L [class] 2 2

[0026] The glass combines high chemical resistance and high thermalshock resistance, especially low thermal expansion, with good meltingproperties, especially a low working point. It is thus superior toborosilicate glasses 3.3 for applications which, although requiring arelatively high thermal shock resistance of the glasses, may not requirethe glasses to comply with DIN ISO 3585, because they can be produced atlower melting points and with higher melting capacities.

[0027] The fact that the glass preferably contains no additionalcomponents, can be a great advantage because it may be producedalternatively with the borosilicate glass 3.3 in the same productionequipment, and only low remelting times occur. The increasedproductivity of the glass melting equipment with this glass reduces theproduction costs of manufacture for some products, particularly, thermalshock-resistant beverage containers that retain the quality of theproperties relevant to this use.

[0028] The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

[0029] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. A glass comprising in % by weight, based onoxide: SiO₂ about 78.5-about 79.5, B₂O₃ about 13.0-about 14.0, Al₂O₃about 2.0-about 3.0, Na₂O about 4.5-about 5.5, K₂O 0-about 0.6, andoptionally at least one fining agent; and Er₂O₃, CoO, or a combinationthereof.
 2. A process for making glass comprising melting together:about 78.5 to about 79.5 weight percent based on oxide SiO₂; about 13.0to about 14.0 weight percent based on oxide B₂O₃; about 2.0 to about 3.0Al₂O₃ weight percent based on oxide; and about 4.5 to about 5.5 Na₂Oweight percent based on oxide; and Er₂O₃, CoO or a combination thereof.3. A glass comprising in % by weight, based on oxide: about 78.5 toabout 79.5 SiO₂; about 13.0 to about 14.0 B₂O₃; about 2.0 to about 3.0Al₂O₃; about 4.5 to about 5.5 Na₂O; and Er₂O₃, CoO, or a combinationthereof.